With a $629.6 billion defense bill heading for the Senate floor, Washington woke up to headlines Monday warning of “mission failure” if more U.S. troops aren’t committed to battle the Taliban.

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Arizona Sen. John McCain, the president’s former Republican rival, demanded that the war’s top brass be called “immediately” to testify before the Senate Armed Services Committee. And down the line, the next 12 months in Afghanistan — running up to the 2010 elections here at home — are being hailed as pivotal.

Obama himself remains cautious about committing more U.S. troops, but for months now, there has been a steady drumbeat of reports from those close to the U.S. commander in Afghanistan, Gen. Stanley McChrystal, that significantly more troops are needed beyond the 21,000 increase approved by the president in February. Monday’s Washington Post, revealing excerpts of McChrystal’s 66-page assessment, capped this campaign and confirmed what some in Congress see as at least “daylight” between the McChrystal camp in the military and the National Security Council team advising Obama.

By Monday night, Republicans had jumped in aggressively, with House Minority Leader John Boehner (R-Ohio) echoing McCain’s demands for public testimony from McChrystal and Gen. David Petraeus, who leads the U.S. Central Command overseeing military operations in both Iraq and Afghanistan.

“Further delay in the decision to send the needed additional forces would endanger the lives of the 68,000 men and women currently serving in Afghanistan,” McCain said. Added Boehner: “The longer we wait, the more we put our troops at risk.”

Senate Armed Services Committee Chairman Carl Levin (D-Mich.) did not immediately respond to McCain’s demands, preferring in his statement to focus on McChrystal’s calls for a significant change in the U.S. strategy — not simply more soldiers and Marines. But the aggressive tone taken by Republicans appeared aimed at two goals.

The first is to make it harder for Obama to delay the troop question when the president would prefer to keep the focus on health care reform.

Second — and this will be delicate politically — is to set down conditions that could make it easier for conservatives to walk away if they feel that Obama is equivocating on his commitment to Afghanistan.

Asked if he thought delay by the president in approving new troops would weaken Republican support, McCain was uncertain. “Too many variables for me to predict,” he told POLITICO. But the whole bitter debate over Iraq — and years of what conservatives saw as costly drift before the military surge in 2007 — hangs over many Republican lawmakers. “I’ve seen this movie before,” McCain said after a Senate hearing last week on the military situation in Afghanistan.

Republican votes are vital to Obama because of the skittishness among Democrats toward the growing military commitment. Even Senate Appropriations Committee Chairman Daniel Inouye (D-Hawaii), a World War II hero with a long record of supporting the Pentagon, has been openly skeptical about what the military can accomplish in Afghanistan. And younger members like Sen. Jim Webb (D-Va.), a Marine and Vietnam veteran who served later in the Defense Department, fear the U.S. is losing precious “maneuverability” in the larger worldwide war against terrorism.

The defense budget bill, due on the Senate floor late this week or early next week, already includes $128.6 billion in contingency funds for military operations in Afghanistan and Iraq for the coming fiscal year. But substantially more would be needed if 20,000 or more troops are added, and Rep. John Murtha (D-Pa.) has warned that a supplemental spending bill could be required next spring — something the White House had sworn to avoid.

“It is a war that the great, great majority of American people supported, and my sense is members are approaching this very thoughtfully,” said Rep. Vic Snyder, a member of the House Armed Services Committee. But the Arkansas Democrat, a veteran of Vietnam, said the single-minded focus on troop levels — by his party or Republicans — was a mistake.

“The complexity doesn’t come from the level of U.S. troops. The complexity comes from the fact that Afghanistan is a very underdeveloped country with a lot of history behind it,” Snyder said. “My own view is we ought to recognize that commanders are going to make requests and we ought to be able to move troop strength up or down without it creating a huge uproar.”

“I’m not trying to minimize the sacrifice of families, who are sending their sons, but I hope — I know the president is looking at more than troop strengths. He has to look at a whole strategy.”

McChrystal, sensitive to civilian casualties, speaks more of a counterinsurgency effort than the counterterrorism approach taken under the Bush administration. But at another level, his sophisticated analysis runs up against time pressures — a 12-month window — which are immense given the complexity of the problems ahead.

Just last month, the Pentagon asked for a temporary increase of 22,000 troops for the Army, for example, but getting those recruits trained and ready can’t be done within 12 months. The rough terrain of Afghanistan has required a whole new fleet of off-road mine-resistant armored vehicles 10,000 pounds lighter than those that could drive on pavement in Iraq.

And the whole challenge of training the Afghan force is extraordinary given the nation’s literacy rate.

Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, admitted the challenge in sometimes eloquent testimony last week before the Senate.

“I’m greatly worried about the time that we have. And that’s why [there is] such a sense of urgency about getting this right. ... And at the same time, it’s almost — I feel we almost must take time because it’s such a vital part of the world.

“So it’s a real conundrum in that regard. I think we have to move quickly to start to turn this thing around. And then, at the same time, I think we have to have a long-term relationship,” he said, telling of going to a school in the Panjshir Province, which had opened the doors to girls getting an education they would never have under the Taliban.

“To grow up and make a difference as they — as they raise families, Mullen said. The admiral said his hope was that the future mothers would help create a less violent world by giving guidance to their sons. ‘You’re not going to do this.’”